Electronic component unit and manufacturing method thereof

ABSTRACT

According to embodiments, there is provided an electronic component unit, including: a circuit board including: a heat generating element that generates a heat; and a bonding metal foil layer formed on a face thereof; a heat transfer board including: a board body having a thermal conductivity higher than that of the circuit board; an insulative layer formed on a first face of the board body; and a heat transfer metal foil layer formed to cover the insulative layer; and a heat sink, wherein the circuit board is assembled with the heat sink via the heat transfer board such that (1) the heat transfer metal foil layer is soldered to the bonding metal foil layer and (2) the second face of the board body is superimposed on the heat sink.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from Japanese Patent Application No.2010-026587 filed on Feb. 9, 2010, the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate to an electronic component unithaving a heat sink for external dissipation of heat from an electroniccomponent (heat generating element) that generates a large amount ofheat due to a large current, and a manufacturing method thereof.

BACKGROUND

In an electronic component unit, various types of electronic componentincluding heat generating elements are included. Examples of heatgenerating elements are semiconductor switching elements, such as powerfield effect transistors (FETs), and shunt resistors. Power FETs may beused to control drive current in motors. In order to efficientlydissipate heat generated by the heat generating elements so as to bewithin their permitted temperature ranges, for example, a heat sink isprovided. For example, in JP-H07-161925-A and JP-UM-3128955-B, a heatsink is employed for external dissipation of heat transferred from theheat generating elements to a circuit board.

In the electronic component unit of JP-H07-161925-A, a metal base boardand a circuit board are separately attached to a heat dissipation case(corresponding to a heat sink). Heat generating elements are mounted tothe metal base board via an insulating layer, and other electroniccomponents are mounted to the circuit board. However, inJP-H07-161925-A, wiring leads for electrical connection between themetal base board and the circuit board are required, thereby increasingthe number of constituent elements and the number of assembly processes.

In the electronic component unit of JP-UM-3128955-B, heat generatingelements are mounted on a circuit board, and a tin layer is provided onthe back face of the circuit board via a copper foil layer. A heatdissipation sheet (corresponding to a heat sink) is laminated on the tinlayer, and fixed to the copper foil layer by melting the tin layer in areflow oven. In JP-UM-3128955-B, the circuit board and the heatdissipation sheet should be electrically insulated. When an insulativemember is simply interposed between the circuit board and the heatdissipation sheet, the number of constituent elements and the number ofassembly processes will be increased.

SUMMARY

Embodiments described herein provide an electronic component unitcapable of efficiently dissipating heat generated by a heat generatingelement, while reducing the number of constituent elements and thenumber of assembly processes, and a manufacturing method thereof.

According to a first aspect of the embodiments, there is provided anelectronic component unit, including: a circuit board including:electronic components mounted on the circuit board, the electroniccomponents including a heat generating element that generates a heat;and a bonding metal foil layer formed on a face of the circuit board; aheat transfer board including: a board body having a thermalconductivity higher than that of the circuit board, the board bodyhaving a first face and a second face opposite to the first face; aninsulative layer formed on the first face; and a heat transfer metalfoil layer formed to cover the insulative layer on the first face; and aheat sink configured to externally dissipate the heat generated in theheat generating element, wherein the circuit board is assembled with theheat sink via the heat transfer board such that (1) the heat transfermetal foil layer on the first face of the board body is soldered to thebonding metal foil layer on the circuit board and (2) the second face ofthe board body is superimposed on the heat sink.

According to a second aspect of the embodiments, there may be providedthe electronic component unit, wherein the board body is a metal flatplate, wherein the insulative layer is an insulative covering filmprinted on the first face of the flat plate, and wherein the heattransfer metal foil layer is a copper foil layer printed to cover thecovering film.

According to a third aspect of the embodiments, there may be providedthe electronic component unit, wherein the bonding metal foil layerincludes a circuit pattern for the heat generating element, wherein theheat transfer metal foil layer has a pattern similar to the circuitpattern, and wherein both the patterns are electrically connected toeach other.

According to a fourth aspect of the embodiments, there may be providedthe electronic component unit, wherein the circuit board has a throughhole therein at a mounting area for mounting the heat generatingelement, and wherein a solder is filled in the through hole so as to beconnected with the bonding metal foil layer.

According to a fifth aspect of the embodiments, there may be providedthe electronic component unit, wherein the circuit board has a pluralityof mounting areas for respectively mounting a plurality of heatgenerating elements, wherein the heat transfer board is divided so as toeach correspond to a respective one of the plurality of mounting areas.

According to a sixth aspect of the embodiments, there is provided anelectronic component unit manufacturing method, including: preparing: acircuit board including a heat generating element mounted thereon and abonding metal foil layer formed thereon; a heat transfer board includingan insulative layer formed on one face thereof and a heat transfer metalfoil layer formed on the insulative layer; and a heat sink; applying acream solder to form a solder layer on the board bonding metal foillayer or the heat transfer metal foil layer; superimposing the bondingmetal foil layer and the heat transfer metal foil layer with each othervia the solder layer; re-melting the solder layer to solder the bondingmetal foil layer and the heat transfer metal foil layer; andsuperimposing the other face of the heat transfer board with the heatsink to thereby assemble the circuit board with the heat sink via theheat transfer board.

In the first aspect, the heat transfer board is interposed between thecircuit board and the heat sink. The heat transfer board has the metalfoil layer for heat transfer formed on the first face of the higherthermal conductivity board body, with the insulative layer interposedtherebetween. The circuit board and the heat sink can be assembled intoa single unit by bonding the heat transfer metal foil layer to the metalfoil layer formed on the circuit board by solder, and superimposing andassembling the second face of the board body to the heat sink.

Since both the heat generating element and the other electroniccomponents can be mounted to the circuit board, no wiring components arerequired for connecting the heat generating element to the otherelectronic components. Furthermore, since the first face of the boardbody has the insulative layer, there is no need for a separate component(such as an insulation sheet) to electrically insulate between thecircuit board and the heat sink. Consequently, while reducing the numberof constituent elements, the number of assembly processes can also bereduced. Since the heat transfer board is interposed between the circuitboard and the heat sink, heat generated by the heat generating elementcan be efficiently externally dissipated with a simple construction bysimply bonding the circuit board and the heat transfer board withsolder.

In the second aspect, the heat transfer board is an integrated componentin which the copper foil layer formed on the first face of the metalflat plate via the insulative covering film. Since the heat transferboard having the metal flat plate as a central member is fixed to thecircuit board, the circuit board can be reinforced. As a result, thebending rigidity of the circuit board can be increased. Consequently,the circuit board and the heat transfer board can be closely bonded tothe heat sink, further increasing the heat dissipation effect from thecircuit board to the heat sink through the heat transfer board.

In the third aspect, the heat transfer metal foil layer is formed on thecircuit board with a pattern similar to the circuit pattern for the heatgenerating element. Both these patterns are electrically connected toeach other. Hence current supplied to the heat generating element canflow through both of the circuit pattern formed on the circuit board andthe pattern of the heat transfer metal foil layer. Since the electricalconducting capacity of the circuit pattern formed on the circuit boardcan be reduced, the thickness and/or width of the circuit pattern can bemade smaller. Furthermore, since electricity is conducted to the heatgenerating element by both patterns, an even larger capacity heatgenerating element can be mounted to the circuit board.

In the fourth aspect the circuit board includes the through hole in themounting area for mounting the heat generating element, and solder isfilled in the through hole so as to be connected with the circuit boardbonding metal foil layer. Accordingly, heat generated by the heatgenerating element can be efficiently transferred to the heat transferboard through the solder filled in the through hole, and thentransferred onward to the heat sink. Consequently, the heat dissipationeffect can be increased even higher.

In the fifth aspect, the circuit board includes plural mounting areasfor mounting the plural heat generating elements. The heat transferboard is divided correspondingly with the plural mounting areas.Consequently, efficient heat dissipation effect is obtained whileminimizing the use of the relatively high cost heat transfer boards.

In the sixth aspect, the solder layer is formed on one of the metal foillayers by applying a cream solder, and the two metal foil layers aresolder bonded to each other by superimposing both of the metal foillayers on each other via the solder layer, and then re-melting thesolder layer. Thus, the heat transfer board is integrated to the circuitboard. Since the heat transfer metal foil layer is formed on the oneface of the heat transfer board via the insulative layer, a separatecomponent (such as an insulating sheet) is not required for electricallyinsulating between the circuit board and the heat transfer board.Consequently, while reducing the number of constituent elements, thenumber of assembly processes can also be reduced.

The other face of the heat transfer board is then superimposed andassembled to the heat sink. The heat transfer board is therebyinterposed between the circuit board and the heat sink, and the threecomponents can thereby be assembled as a single unit. Accordingly, heatgenerated by the heat generating element can be efficiently externallydissipated by simply interposing the heat transfer board between thecircuit board and the heat sink and bonding the circuit board and theheat transfer board together with solder.

Furthermore, since both the heat generating element and the otherelectronic components can be mounted to the circuit board, no wiringcomponents are required for connecting the heat generating element tothe other electronic components. Consequently, while reducing the numberof constituent elements, the number of assembly processes can also bereduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 cross-sectionally illustrates an electronic component unitaccording to a first Embodiment.

FIG. 2 is an exploded diagram of the electronic component unitillustrated in FIG. 1.

FIG. 3 enlargedly illustrates the portion 3 in FIG. 1.

FIG. 4A to 4C illustrate each of the circuit patterns of the circuitboard and the heat transfer board in FIG. 2, and a layout for componentmounting to the circuit board.

FIG. 5 is an exploded diagram of the circuit board and the heat transferboard in FIG. 2, as viewed from the heat transfer board side.

FIG. 6 illustrates a manufacturing method of the electronic componentunit in FIG. 1.

FIG. 7 cross-sectionally illustrates an electronic component unitaccording to a second Embodiment.

FIG. 8 cross-sectionally illustrates an electronic component unitaccording to a third Embodiment.

FIG. 9 cross-sectionally illustrates an electronic component unitaccording to a fourth Embodiment.

DETAILED DESCRIPTION

Embodiments will be described, with reference to the attached drawings.

First Embodiment

An electronic component unit according to a first Embodiment will bedescribed, with reference to FIG. 1 to FIG. 5. As shown in FIG. 1 andFIG. 2, an electronic component unit 10 of the first Embodiment includesa circuit board 20, a heat transfer board 30, a heat sink 40 and a cover50.

The circuit board 20 is a flat board formed of an insulative resinmaterial, such as an epoxy resin. On the circuit board 20, electricalcomponents 60 and connectors 71 are provided. The electrical components60 include heat generating elements 61 and other elements 62. Accordingto the Embodiments, as the electrical components 60, at least one heatgenerating element 61 is attached to the circuit board 20 by surfacemounting.

For example, the “heat generating elements” 61 include a semiconductorswitch element and a shunt resistor in which a comparatively largeamount of heat is generated due to a large current. An example of thesemiconductor switch element is a power FET. For example, the power FETmay be used to drive a motor in an electrical power assisted steeringsystem. For example, the other elements 62 include a capacitor.

“Surface mounting” is a method for attaching/mounting an electroniccomponent to a circuit board by directly soldering the electroniccomponent to the surface of the circuit board. According to the surfacemounting, mounting density can be increased as compared with a method inwhich leads of electronic components are fixed into holes in a circuitboard.

Specifically, in the “surface mounting”, a cream solder is first printedonto a base board at positions for mounting electronic components.Electronic components are then mounted at the mounting positions using achip mounter. The base board mounted with the electronic components isthen placed in a reflow oven, so as to melt the solder with heat (areflow process) to thereby fix the electronic components to the baseboard.

In this embodiment, as shown in FIG. 3, a so-called double sided board(two-layer board) is used as the circuit board 20, and metal foil layers21, 22 are formed on either face of the circuit board 20. In the circuitboard 20, the first metal foil layer 21 is formed on a first board face20 a, and the second metal foil layer 22 is formed on a second boardface 20 b. The second board face 20 b faces the heat transfer board 30,and the first board face 20 a is opposite to the heat transfer board 30.The circuit board 20 and the first and second metal foil layers 21, 22form a laminated structure 23. The circuit board is not limited to atwo-layer board, but may be any multi-layer board.

FIG. 4A shows the circuit board 20 on which the electrical components 60and the connectors 71 are mounted, as viewed from the first board face20 a side. FIG. 4B shows the circuit board 20 alone, as viewed from thefirst board face 20 a side. FIG. 4C shows the heat transfer board 30, asviewed from a first board face 31 a side.

As shown in FIG. 4A, 4B and FIG. 5, the first and second metal foillayers 21, 22 are shaped into thin film. The first and second metal foillayers 21, 22 are printed on the circuit board 20 to have specificcircuit patterns. The laminated structure 23 (the first metal foil layer21, the circuit board 20 and the second metal foil layer 22) ispartitioned into a first area 24 and a second area 25. The heatgenerating elements 61 are mounted on the first area 24 of the firstmetal foil layer (first circuit pattern layer) 21, and the otherelements 62 are mounted on the second area 25 of the first metal foillayer (first circuit pattern layer) 21. The second metal foil layer(second circuit pattern layer) 22 is employed for bonding to the heattransfer board 30.

As shown in FIGS. 4A and 4B, the first area 24 of the first circuitpattern layer 21 includes mounting areas 24 a for mounting therespective heat generating elements 61. The heat generating elements 61are surface mounted at the respective mounting areas 24 a so as to beelectrically connected to the first circuit pattern layer 21.Specifically, as shown in FIG. 3, a first solder layer 26 is formed onthe first circuit pattern layer 21 by printing, and the heat generatingelements 61 are surface mounted to the first circuit pattern layer 21 byre-melting the first solder layer 26. The other elements 62 are alsomounted to the first circuit pattern layer 21 through the first solderlayer 26.

As shown in FIG. 4A, holes are formed in the circuit board 20 at thesecond area 25. The other elements 62 are electrically connected to thefirst circuit pattern layer 21 by inserting leads of the other elements62 into the holes in the circuit board 20 and soldering.

As shown in FIG. 5, at the first area 24, the second circuit patternlayer 22 is formed so as to correspond to the first circuit patternlayer 21 (see FIG. 4B). Namely, the second circuit pattern layer 22 isformed with substantially the same pattern as the first circuit patternlayer 21 within the first area 24.

As shown in FIG. 3 and FIG. 5, the heat transfer board 30 overlaps atleast partially with the second circuit pattern layer 22 on the circuitboard 20. The heat transfer board 30 includes a board body 31, aninsulative layer 32 and a metal foil layer 33. The insulative layer 32is formed on the first board face 31 a of the board body 31, and themetal foil layer 33 is formed on the top face (outside face) of theinsulative layer 32 for heat transfer.

The board body 31 is formed of a material having a higher thermalconductivity than the circuit board 20. Specifically, the board body 31is shaped into a flat plate and formed of a metallic material such as analuminum or copper alloy. For example, at a temperature of 0° C., thethermal conductivity of aluminum is 236 W/mK, the thermal conductivityof copper is 403 W/mK, and the thermal conductivity of bisphenol A epoxyresin is 0.21 W/mK.

The heat transfer board 30 is generally referred to as a metal baseboard as it has the metallic board body 31. For example, when the boardbody 31 is formed of an aluminum alloy, the heat transfer board 30 maybe referred to as an aluminum base board (aluminum board).

As shown in FIG. 3, the insulative layer 32 is a covering film formed byprinting to cover substantially the entire first board face 31 a of theboard body 31. The insulative layer 32 has a thin film shape and formedof a material that is both electrical insulation property and highthermal conductivity, such as an epoxy resin. In order to increase heatdissipation ability while maintaining electrical insulation property,for example, the thickness of the insulative layer 32 is set in therange from 50 μm to 200 μm. When the insulative layer 32 is too thin,the electrical insulation property may be deteriorated, and when theinsulative layer 32 is too thick, the heat dissipation ability may besuppressed low. It is preferable to use the insulative layer 32 havingthe high heat transfer ability to maintain the heat transfer ability tothe heat transfer board 30 (to maintain good heat dissipation ability).

As shown in FIG. 3 and FIG. 4C, the heat transfer metal foil layer 33 isa thin film shaped conductive layer, and is disposed on the first boardface 31 a of the board body 31 via the insulative layer 32. The metalfoil layer 33 is formed of a copper foil layer printed onto the top faceof the covering film 32, and is formed to have a similar pattern to thesecond circuit pattern layer 22. The metal foil layer (third circuitpattern layer) 33 is provided for heat transfer, and no component ismounted to the metal foil layer 33.

The heat transfer metal foil layer (third circuit pattern layer) 33 ofthe heat transfer board 30 is bonded to the second metal foil layer(second circuit pattern layer) 22 of the laminated structure 23 by asecond solder layer 34. The second solder layer 34 is formed to have asimilar pattern to that of the second and third circuit pattern layers22, 33. Thus, both the circuit pattern layers 22, 33 are electricallyconnected to each other through the second solder layer 34.

As shown in FIG. 1 to FIG. 3, the heat sink 40 is a heat dissipationplate for externally dissipating heat transferred to the circuit board20 from the heat generating elements 61, and also serves as a base ofthe electronic component unit 10. The heat sink 40 is, for example,formed of an aluminum or copper alloy. A second board face 31 b of theboard body 31 (the board face 31 b opposite to the circuit board 20)overlaps with a first face 40 a of the heat sink 40 and is attached tothe heat sink 40 by plural fitting members 41, such as screws or rivets.

As shown in FIG. 3, the second board face 31 b of the heat transferboard 30 is preferably joined to the first face 40 a of the heat sink 40via a heat dissipation grease layer 35, such as a silicone grease. Heattransfer from the heat transfer board 30 to the heat sink 40 can bepromoted by interposing the heat dissipation grease layer 35therebetween.

The heat transfer board 30 and the laminated structure 23 (circuit board20) are thus formed on top of heat sink 40 in this sequence, andfastened with the common fitting members 41 to thereby form a singleunit. The laminated structure 23 (circuit board 20) and the heattransfer board 30 assembled to the heat sink 40 are covered by the cover50.

The heat generated by at least the heat generating elements 61 istransferred to the heat sink 40 through the first solder layer 26, thefirst circuit pattern layer 21, the circuit board 20, the second metalfoil layer (second circuit pattern layer) 22, the second solder layer34, the heat transfer metal foil layer (third circuit pattern layer) 33,the insulative layer 32, the board body 31 and the heat dissipationgrease layer 35. And, the transferred heat is then dissipated from theheat sink 40 to the atmosphere.

As shown in FIG. 5, the second circuit pattern layer 22 on the circuitboard 20 has an area for mounting a heat generating element 63 (such asa shunt resistor) and an area for mounting other electronic components,at locations not overlapping with the heat transfer board 30. The heatgenerated by the heat generating element 63 is transferred to the heatsink 40 through the second metal foil layer (second circuit patternlayer) 22, the second solder layer 34, the heat transfer metal foillayer (third circuit pattern layer) 33, the insulative layer 32, theboard body 31 and the heat dissipation grease layer 35. And, thetransferred heat is then dissipated from the heat sink 40 to theatmosphere.

In the electronic component unit 10 according to the first embodiment,the heat transfer board 30 is interposed between the laminated structure23 (circuit board 20) and the heat sink 40. The heat transfer board 30includes the board body 31 of high thermal conductivity and the heattransfer metal foil layer 33 formed on the first board face 31 a via theinsulative layer 32. The laminated structure 23 (circuit board 20) andthe heat sink 40 can be assembled into a single unit by bringing thesecond board face 31 b of the board body 31 onto the first face 40 a ofthe heat sink 40 and by soldering/bonding the heat transfer metal foillayer 33 of the heat transfer board 30 to the second metal foil layer 22of the laminated structure 23.

Since both the heat generating elements 61 and the electrical components60 can be mounted to circuit board 20, wiring for connecting the heatgenerating elements 61 and the other elements 62 is not required. Sincethe insulative layer 32 is provided to the first board face 31 a of theboard body 31, a separate component (such as an insulating sheet) isalso not required for electrically insulating between the circuit board20 and the heat sink 40. Consequently, both of the number of constituentelements and the number of assembly processes can be reduced. Further, aconfiguration for externally dissipating the heat generating elements 61can be simply accomplished by interposing the heat transfer board 30between the circuit board 20 and the heat sink 40, and by bonding thecircuit board 20 and the heat transfer board 30 with solder.

The heat transfer board 30 is a single component in which the metal foillayer 33 is formed to the first board face 31 a of the metal flat board31 (board body 31) via the insulative covering film 32 (insulative layer32). Since the metal flat board 31 as a main component of the heattransfer board 30 is fixed to the circuit board 20, the strength of thecircuit board 20 can be increased by the heat transfer board 30. As aresult, the bending rigidity of the circuit board 20 is increased.Consequently, the circuit board 20 and the heat transfer board 30 can beclosely bonded to the heat sink 40, further increasing the heatdissipation effect from the circuit board 20 to the heat sink 40 throughthe heat transfer board 30.

The manufacturing method of the electronic component unit 10 of thefirst Embodiment will be described, with reference to FIG. 1 to FIG. 5,and FIG. 6.

In the manufacturing method of the electronic component unit 10, first,in step S1 (preparatory process) shown in FIG. 6, the circuit board 20,the heat transfer board 30 and the heat sink 40 are prepared. The firstcircuit pattern layer 21 is formed on the first board face 20 a of theprepared circuit board 20, and the second circuit pattern layer 22 isformed on the second board face 20 b thereof. When preparing the heattransfer board 30, the insulative layer 32 is formed on the first boardface 31 a of the board body 31, and the heat transfer metal foil layer33 is formed on the insulative layer 32.

At step S2 (the second solder layer forming process) subsequent to thepreparatory processes, the second solder layer 34 is formed by applyinga cream solder to the board bonding second metal foil layer 22 or to theheat transfer metal foil layer 33. A cream solder printer is used toapply a cream solder to one of the metal foil layers 22, 33 to therebyform the second solder layer 34. The cream solder is a solder in a creamstate prepared from particles of solder in solvent and flux.

At a step S3 (first mounting process) subsequent to the second solderlayer forming process, the board bonding second metal foil layer 22 andthe heat transfer metal foil layer 33 are superimposed on each otherwith the second solder layer interposed therebetween. At the same time,the heat generating element 63 (see FIG. 5), such as a shunt resistor,is mounted to the board bonding second metal foil layer (second circuitpattern layer) 22 with the second solder layer 34 interposedtherebetween.

At a step S4 (first bonding process) subsequent to the first mountingprocess, the board bonding second metal foil layer 22 and the heattransfer metal foil layer 33 are bonded by soldering by re-melting thesecond solder layer 34. For example, the circuit board 20 in a mountedstate with the heat transfer metal foil layer 33 and the heat generatingelement (shunt resistor) 63 is placed in a reflow oven, heated, and thencooled (reflow process). As a result, the second solder layer 34 bondsboth the metal foil layers 22, 33 to each other by re-melting and thensolidifying. The board bonding second metal foil layer 22 is also bondedto the heat generating element (shunt resistor) 63 at the same time.

At a step S5 (first solder layer forming process) subsequent to thefirst bonding process, the circuit board 20 is inverted, and the firstsolder layer 26 is formed by applying a cream solder to the firstcircuit pattern layer 21. The first solder layer 26 is, similarly to thesecond solder layer 34, formed by printing (applying) a cream solder onthe first circuit pattern layer 21 using a cream solder printer.

At a step S6 (second mounting process) subsequent to the first solderlayer forming process, the heat generating elements 61 are mounted tothe first circuit pattern layer 21 on the circuit board 20, with thefirst solder layer 26 interposed therebetween.

At a step S7 (second bonding process) subsequent to the second mountingprocess, the circuit board 20 is placed in a reflow oven with the heatgenerating elements 61 in an mounted state, heat is applied, and thencooling is performed (reflow process). As a result, the heat generatingelements 61 are bonded to the first circuit pattern layer 21 by thefirst solder layer 26 re-melting and then solidifying.

At a step S8 (third mounting process) subsequent to the second bondingprocess, the other elements 62 are mounted by inserting the leadsthereof into the lead attachment holes formed in the circuit board 20through the first circuit pattern layer 21.

At a step S9 (third bonding process) subsequent to the third mountingprocess, the circuit board 20 is passed through a flow tank with theother elements 62 in a mounted state, and solder is applied. As aresult, the other elements 62 are bonded to the first circuit patternlayer 21 on the circuit board 20 by soldering.

At a step S10 (assembly process) subsequent to the third bondingprocess, the second board face 31 b of the heat transfer board 30 issuperimposed on the heat sink 40, and the heat transfer board 30 and thecircuit board 20 are assembled to the heat sink 40 with the fittingmembers 41. The heat sink 40 is then covered by the cover 50, thuscompleting manufacture of the electronic component unit 10.

Note that the execution sequence for executing steps S2 to S4 and forexecuting steps S5 to S7 may be swapped over.

In the manufacturing method of the electronic component unit 10according to the first embodiment, the solder layer (second solderlayer) 34 (is formed by applying a cream solder to one of the metal foillayers 22, 33, both the metal foil layers 22, 33 are superimposed oneach other with the second solder layer 34 interposed therebetween, andby re-melting the second solder layer 34, the metal foil layers 22, 33are bonded together by soldering. As a result, the heat transfer board30 is integrated to the circuit board 20. Since the heat transfer metalfoil layer 33 is formed on the first board face 31 a of the heattransfer board 30 via the insulative layer 32, a separate component(such as an insulating sheet) for electrical insulation between thecircuit board 20 and the heat transfer board 30 is not required.Consequently, while reducing the number of constituent elements, thenumber of assembly processes can also be reduced.

Next, the second board face 31 b of the heat transfer board 30 issuperimposed and assembled to the first face 40 a of the heat sink 40.The heat transfer board 30 is thereby interposed between the circuitboard 20 and the heat sink 40, and the three components 20, 30, 40 canthereby be assembled as a single unit. Accordingly, by simplyinterposing the heat transfer board 30 between the circuit board 20 andthe heat sink 40 and bonding the circuit board 20 and the heat transferboard 30 together with solder, heat generated by the heat generatingelements 61 can be efficiently externally dissipated.

Since both the heat generating elements 61 and the other electricalelements 62 can be mounted to the circuit board 20, wiring forconnecting the heat generating elements 61 and the other elements 62 isnot required. Consequently, while reducing the number of constituentelements, the number of assembly processes can also be reduced.

Second Embodiment

An electronic component unit according to a second Embodiment will bedescribed. FIG. 7 cross-sectionally illustrates an electronic componentunit 10A of the second Embodiment. In the second Embodiment, theelectronic component unit 10 shown in FIG. 3 is modified into anelectronic component unit 10A shown in FIG. 7. Since other parts of theconfiguration and the manufacturing method are similar to those shown inFIG. 1 to FIG. 6 further explanation is omitted.

Specifically, in the electronic component unit 10A of the secondEmbodiment, the circuit board 20 has at least one through hole 81provided in a mounting area 24 a (see FIG. 4B) for mounting a heatgenerating element 61. The through hole 81 passes through the circuitboard 20 in the thickness direction. Ring shaped metal foil 82 is formedto electrically connect the wall faces of the through hole 81 to thefirst and second metal foil layers 21, 22. For example, the ring shapedmetal foil 82 is formed together with the first and second metal foillayers 21, 22 by printing or electroplating the wall faces of thethrough hole 81. The ring shaped metal foil 82 is connected to the firstand second metal foil layers 21, 22 such that the first and second metalfoil layers 21, 22 are electrically connected to each other.

The heat transfer metal foil layer 33 is formed to the circuit board 20as described above, with a similar pattern to the circuit pattern of thefirst and second metal foil layers 21, 22. The circuit patterns of thefirst and second metal foil layers 21, 22 are electrically connected tothe pattern of the heat transfer metal foil layer 33 through the secondsolder layer 34. Current to be supplied to the heat generating elements61 can thus flow through both the circuit pattern of the first andsecond metal foil layers 21, 22 and the circuit pattern of the heattransfer metal foil layer 33. Namely, the heat transfer metal foil layer33 can be efficiently utilized.

Consequently, the electrical conducting capacity of the circuit patternof the first metal foil layer 21 can be reduced. Accordingly, thethickness and/or width of the circuit pattern of the first metal foillayer 21 can be reduced by this amount.

The large capacity heat generating element 61 can be mounted to thecircuit board 20, by supplying current to the heat generating element 61from the circuit pattern of the first metal foil layer 21, from thecircuit pattern of the second metal foil layer 22 and from the circuitpattern of the heat transfer metal foil layer 33.

Solder 83 (in-hole solder 83) is filled in the through hole 81.Specifically, the in-hole solder 83 is filled into the ring shaped metalfoil 82. The in-hole solder 83 is connected to the first and secondmetal foil layers 21, 22 and the first and second solder layers 26, 34,such that the in-hole solder 83 electrically connects these together.

Heat generated by the heat generating elements 61 is transferred withgood efficiency to the heat transfer board 30 through the solder(in-hole solder) filled in the through hole 81, and then transferredonward to the heat sink 40. Consequently, the heat dissipation abilitycan be increased further.

The electronic component unit 10A of the second Embodiment has similaroperation and effect to that of the electronic component unit 10 of thefirst Embodiment.

Third Embodiment

An electronic component unit of a third Embodiment will be described.FIG. 8 cross-sectionally illustrates an electronic component unit 10B ofthe third Embodiment. In the third Embodiment, the electronic componentunit 10A of the second Embodiment shown in FIG. 7 is modified into theelectronic component unit 10B shown in FIG. 8. Since other parts of theconfiguration and the manufacturing method are similar to those shown inFIG. 1 to FIG. 6 further explanation is omitted.

In FIG. 8, plural heat generating elements 61, 61 are mounted to thecircuit board 20, with the circuit pattern of the first metal foil layer21, the circuit pattern of the second metal foil layer 22 and thecircuit pattern of the heat transfer metal foil layer 33 separatelycorresponded to each of the heat generating elements 61, 61. There is atleast one through hole 81 in the circuit board 20 for each of therespective heat generating elements 61, 61. Accordingly, the throughholes 81 corresponding to each of the respective heat generatingelements 61, 61 are electrically isolated from each other.

The electronic component unit 10B of the third Embodiment has similaroperation and effect to the electronic component unit 10 of the firstEmbodiment.

An electronic component unit according to a fourth Embodiment will bedescribed. FIG. 9 cross-sectionally illustrates an electronic componentunit 10C of the fourth Embodiment. In the fourth Embodiment, theelectronic component unit 10 of the first Embodiment shown in FIG. 1 andFIG. 3 is modified into the electronic component unit 10C shown in FIG.9. Since other parts of the configuration and the manufacturing methodare similar to those shown in FIG. 1 to FIG. 6 further explanation isomitted.

On the circuit board 20, plural mounting areas 24 a (see FIG. 4B) areprovided for mounting plural heat generating elements 61. The heattransfer board 30 is partitioned so as to correspond to each of theplural respective mounting areas 24 a. Namely, the plural heat transferboards 30 need only be small in size, so as to match the size of thecorresponding respective mounting areas 24 a. Depressions 91 are formedin the first face 40 a of the heat sink 40, except in the locationssuperimposed by the respective heat transfer boards 30. Accordingly,efficient heat dissipation effect is obtained while minimizing the useof the relatively high cost heat transfer boards 30.

The electronic component unit 10C of the fourth Embodiment has similaroperation and effect to the electronic component unit 10 of the firstEmbodiment.

The embodiments described herein are not limited to a double sided boardcircuit board 20 having the metal foil layers 21, 22 on both faces, anda single sided board having the second metal foil layer 22 only on thesecond board face 20 b side may be used. In this case, the electricalcomponents 60 including the heat generating element 61 may be mounted tothe second metal foil layer 22.

For example, the electronic component units and manufacturing methods ofthe embodiments exemplified herein are applied for external dissipationof heat generated by heat generating elements, such as semiconductorswitching elements for controlling drive current of a motor in a vehicleelectrically driven power steering apparatus and power elements foraudio applications.

The invention claimed is:
 1. An electronic component unit, comprising: acircuit board on which electronic components are mounted, the electroniccomponents including a heat generating element that generates a heat; abonding metal foil layer formed on a face of the circuit board; a heattransfer board arranged so that the bonding metal foil layer isinterposed between the heat transfer board and the circuit board, theheat transfer board including: a board body having a thermalconductivity higher than that of the circuit board, the board bodyhaving a first face and a second face opposite to the first face; aninsulative layer, having a thin film shape, formed on the first face;and a heat transfer metal foil layer formed to cover the insulativelayer on the first face, the insulative layer being interposed betweenthe board body and the heat transfer metal foil layer; a solder layerbetween the heat transfer metal foil layer and the bonding metal foillayer; and a heat sink configured to externally dissipate the heatgenerated in the heat generating element, wherein the circuit board isassembled with the heat sink via the heat transfer board such that (1)the heat transfer metal foil layer on the first face of the board bodyis soldered to the bonding metal foil layer on the circuit board by thesolder layer, and (2) the second face of the board body is superimposedon the heat sink.
 2. The electronic component unit of claim 1, whereinthe board body is a metal flat plate, wherein the insulative layer is aninsulative covering film printed on the first face of the flat plate,and wherein the heat transfer metal foil layer is a copper foil layerprinted to cover the covering film.
 3. The electronic component unit ofclaim 1, wherein the bonding metal foil layer includes a circuit patternfor the heat generating element, wherein the heat transfer metal foillayer has a pattern similar to the circuit pattern, and wherein both thepatterns are electrically connected to each other.
 4. The electroniccomponent unit of claim 1, wherein the circuit board has a through holetherein at amounting area for mounting the heat generating element, andwherein a solder is filled in the through hole so as to be connectedwith the bonding metal foil layer.
 5. The electronic component unit ofclaim 1, wherein the circuit board has a plurality of mounting areas forrespectively mounting a plurality of heat generating elements, andwherein the heat transfer board is divided so as to each correspond to arespective one of the plurality of mounting areas.
 6. The electroniccomponent unit of claim 1, wherein the circuit board is made of aninsulative resin material.
 7. The electronic component unit of claim 1,wherein the bonding metal foil layer has a thin film shape, and whereinthe heat transfer metal foil layer has a thin film shape.
 8. Theelectronic component unit of claim 1, wherein the insulative layer has athickness of from 50 μm to 200 μm.